Device and a method for thermal sensing

ABSTRACT

A device for thermal sensing is disclosed based on only one thermopile. The cold junctions of said thermopile are coupled thermally to a first channel comprising a first substance while the hot junctions of said thermopile are coupled thermally to a second channel comprising a second substance, said first and said second channel are separated and thermally isolated one from another. Said device can further comprise a membrane to thermally and electrically isolate said thermopile and to mechanically support said thermopile. Particularly a liquid rubber, i.e. ELASTOSIL LR3003/10A, B can be used as a membrane material. Further disclosed is a method for fabricating such a device using micromachining techniques.

FIELD OF THE INVENTION

[0001] The present invention is related to a device which yields anelectrical output signal but has an input or intermediate signal of thethermal type. Such a device can be used to characterize chemical andphysical processes which are accompanied by changes in heat content orenthalpy. Furthermore a method is disclosed for manufacturing saiddevice by means of micromachining which is a technique closely relatedto the technique used for the manufacturing of integrated circuits.

BACKGROUND OF THE INVENTION

[0002] New approaches in the combinatorial chemistry have resulted inthe capability of producing millions of compounds in a short time.Analysis of each compound with respect to multiple parameters is provingto be a significant bottleneck as in e.g. M. A. Shoffner et al., NucleicAcids Research, 1996, vol. 24, No. 2, pp. 375-9. The number of cells,the test reagent volumes, the throughput rate and the ease of usethrough automation are all important parameters which should beoptimized in order to meet the stringent requirements for modem drugscreening. Furthermore a small amount of precious reagent reduces bothcost and waste, and increases the number of possible analyses. Acandidate for this kind of analysis is a calorimeter. A calorimeter is adevice which yields an electrical output signal but has an input orintermediate signal of the thermal type. Calorimetry, more thanpH-metry, offers the advantage of generality: all chemical and physicalprocesses are accompanied by changes in heat content, or enthalpy. Infact microcalorimeters can be used for the analysis of the activity ofbiological cells, chemical reactions in small volumes and othermicroanalytical applications.

[0003] The most frequently used commercially available calorimeters arethe Thermometric 2277 Thermal Activity Monitor and the MicroCal MCSIsothermal Titration Calorimeter. They are both based on the use of twoor more thermoelectric devices, so called thermopiles, having a commonheat sink as reference. A thermopile is at least one thermocouple whichis a temperature sensing element and which is connected to identicalthermocouples in parallel thermally and in series electrically.Thermocouples do not measure the temperature itself, but rather thetemperature difference between two junctions. An advantage of usingthermocouples as temperature sensing elements is that there is nooffset, i.e. when there is no temperature difference there is novoltage, which makes calibration superfluous. A thermocouple asillustrated in FIG. 2, i.e. a combination of two different(semi)conductive materials, converts a thermal difference between itstwo junctions into a voltage difference by means of the combined Seebeckcoefficient S of its two structural thermoelectric materials. In fact athermocouple comprises a first conductive material (14) and a secondconductive material (13) with an insulating layer (15) inbetween. Athermocouple has a so-called hot junction (11), where said firstmaterial and said second material are short-circuited, and a so-calledcold junction (15), where said first and said second material areseparated one from another by means of said insulating layer. At saidcold junction the electrical output signal, representing the temperaturedifference ΔT between said hot junction and said cold junction, can bemeasured.

[0004] The total generated voltage is the sum of the thermocouplevoltages. For n (n being a positive whole number greater than zero)thermocouples, where each thermocouple is identical, it can be writtenthat:

U _(tp) =n*S*ΔT.

[0005] The temperature difference ΔT is the product of the generatedpower difference between the two junction sites and the thermalresistance:

ΔT=ΔP _(gen) *R _(th)

[0006] Thermopiles are preferred because they are self-generating, easyto integrate and because the temperature changes involved are lowfrequency signals.

[0007] The drawbacks of these state-of-the art devices are thefollowing. These devices have at least two thermopiles and a common heatsink. The cold junctions of each thermopile are thermally coupled to thecommon heat sink which is at a known temperature. The hot junctions ofeach thermopile are thermally coupled to a substance under test. So infact, one tries to perform a kind of absolute measurement by measuringthe temperature difference between this substance under test and theheat sink at known temperature. By applying different substances undertest to different thermopiles as e.g. for drug screening where the hotjunctions of a first thermopile are coupled to reference cells and thehot junctions of a second thermopile are coupled to geneticallyengineered cells expressing a drug target. When the potential drugcandidate is effective, it will activate the genetically engineeredcells which results in a heat change. This heat change is determinedindirectly by subtracting the measured signals of the first and thesecond thermopile, where the cold junctions of both thermopiles arecoupled to a common heat sink at known temperature. This is a cumbersomeapproach which lacks accuracy and demands a space consuming design.

SUMMARY OF THE INVENTION

[0008] In an aspect of the invention a device is disclosed based on onlyone thermopile wherein said thermopile is in contact with at least partsof a substrate, e.g. a silicon wafer or the remains thereof. The coldjunctions of said thermopile are coupled thermally to a first channelcomprising a first substance while the hot junctions of said thermopileare coupled thermally to a second channel comprising a second substance,said first and said second channel are separated and thermally isolatedone from another. Said device is capable of handling a very small amountof a substance, typically in the range from 1 microliter to 30microliter.

[0009] In an aspect of the invention a device for monitoring chemicaland physical processes which are accompanied by changes in heat contentor enthalpy is disclosed, comprising a thermopile, wherein saidthermopile is in contact with at least parts of a substrate, e.g. asilicon wafer or the remains thereof, and wherein said thermopile is aset of at least one thermocouple comprising a first conductive materialand a second conductive material with an insulating layer inbetween.Said first and said second material are chosen such that theirthermoelectric voltages are different. A first substance, i.e. areference substance, can be thermally coupled to the cold junctions ofsaid thermopile while a second substance, i.e. a test substance, can bethermally coupled to the hot junctions of the same thermopile.Alternatively, a first substance, i.e. a test substance, can bethermally coupled to the cold junctions of said thermopile while asecond substance, i.e. a reference substance, can be thermally coupledto the hot junctions of the same thermopile. To speed up measurementtime or to test a number of substances at the same time, a modularsystem comprising an array of devices, each device comprising onethermopile, can be configured on the same substrate. Said device canfurther comprise a thin insulating layer, e.g. an oxide layer or anitride layer, on said thermopile in order to prevent a direct contactbetween the substances and the thermopile to thereby avoid damaging saidthermopile. Said device further comprises a membrane to thermally andelectrically isolate said thermopile and to mechanically support saidthermopile. Silicon oxide and/or silicon nitride can be used as membranematerials. Particularly a liquid rubber, i.e. ELASTOSIL LR3003/10A, Bcan be used as a membrane material.

[0010] In an aspect of the invention a method is disclosed forfabricating a device used to monitor chemical and physical processeswhich are accompanied by changes in heat content or enthalpy. The deviceis capable of handling a very small amount of a substance. Theserequirements can be achieved by micromachining, a technique closelyrelated to integrated circuit fabrication technology. The startingmaterial is a substrate like e.g. a semiconductive wafer, particularly amonocrystalline silicon wafer, or a slice of an insulating material,i.e. a glass slice. On this substrate layers can be coated, patterned bymeans of a sequence of lithographic steps and wet and/or dry etchingsteps. Such processed substrates can be bonded to each other or to othermaterials in order to make three-dimensional structures.

[0011] According to the invention a method is disclosed for fabricatingrubber membranes. This method comprises the following steps:

[0012] On a first side of a substrate a silicon oxide/silicon nitridestack is deposited which will serve as an etch mask to define themembrane pattern. Other materials and/or other thickness and or anothernumber of layers may be used to serve as an etch mask. Said substratecan be a semiconductive wafer or slice, like e.g. a silicon wafer, or aninsulating slice like e.g. a glass slice.

[0013] On the second side of the substrate a silicon oxide/siliconnitride stack is deposited which will serve as an etch stop to definethe membrane pattern. Other, preferably insulating, materials and/orother thickness and or another number of layers may be used to serve asan etch stop. One can also choose to omit this etch stop dependent onthe etch procedure.

[0014] The etch mask on said first side of the substrate is patterned bymeans of a sequence of photolithographic steps and wet and/or dryetching steps.

[0015] The second side of the substrate is coated with liquid rubber,i.e. ELASTOSIL LR3003/10A, B. The relatively low viscosity of saidrubber allows for a spin-coating technique. The surface of the substrateis chemically modified to make it water repellent by treating saidsurface with hexamethyldisiloxane (HMDS).

[0016] A second substrate is bonded onto the first by means of theunvulcanised rubber. The bonding is performed in low vacuum and therubber is cured. Alternatively, instead of a second wafer a glass plateis used. A first side of this glass plate comprises a wax layer toprotect the rubber layer of the first substrate because said first sideis exposed to the rubber during the bonding process.

[0017] To free the membrane a chemical back etch is performed in KOH.

[0018] In an aspect of the invention a method is disclosed forfabricating a device used to monitor chemical and physical processeswhich are accompanied by changes in heat content or enthalpy. Thismethod or process comprises the following steps:

[0019] On a first side of a substrate at least one hard mask layer isdeposited which will serve as an etch mask for removing at least partsof said substrate.

[0020] On the second side of the substrate at least one hard mask layercan be deposited which can serve as an etch stop layer dependent on theetch procedure used and/or as an insulating layer to thermally andelectrically isolate a thermopile and/or to inhibit a direct contactbetween a substance and said thermopile.

[0021] On said second side of the substrate a first conductive layerwith a thickness typically in the range from 0.3 μm to 1 μm isdeposited. Said first conductive layer, e.g. a doped polysilicon layer,is patterned to thereby form the first material of the thermopile, i.e.a set of thermocouples which are connected in parallel thermally and inseries electrically.

[0022] On said first side of the substrate said hard mask layer ispatterned in order to define the etch windows for etching away theunderlying silicon in order to expose at least parts of the underlyingthermopile or the etch stop multi-layer structure on said thermopile.

[0023] On said second side of the substrate an insulating layer isdeposited with a thickness in the range typically from 0.2 μm to 1 μm orfrom 0.5 μm to 5 μm. Said insulating layer is used as an interconductivelayer dielectric and isolates the different fingers of the pattern ofthe first conductive layer from each other. Said insulating layer ispatterned to thereby form via holes through which the underlying firstconductive layer can be contacted in order to form hot junctions.

[0024] A second conductive layer, having a thermoelectric voltagedifferent from the thermoelectric voltage of said first conductivelayer, i.e. an aluminum layer with a thickness of 200 nm, is depositedon said second side of the substrate. Said second conductive layer ispatterned to thereby form the second material of the thermopile.

[0025] On said second side of the substrate an insulating layer isdeposited to serve as a membrane. Said membrane should thermally andelectrically isolate said thermopile and mechanically support saidthermopile. Silicon oxide and/or silicon nitride can be used as membranematerials, but preferably a liquid rubber, i.e. ELASTOSIL LR3003/10A, Bis used.

[0026] A second substrate can be bonded or glued onto said firstsubstrate.

[0027] To expose the thermopile and the membrane, a chemical back etchis performed. Said back etch can be a sequence of etching steps usingKOH as an etchant and wherein each etching step is performed with aweight percentage of KOH in the range from 20 to 60 percent and at atemperature in the range from 20 to 100° C. Alternatively an etch mask,e.g. a silicon oxide/silicon nitride stack, can be deposited andpatterned on the free surface of said second substrate. By doing so,during the back etch e.g. only the glass or silicon underneath thethermopile is removed to thereby free the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 depicts two schematic representations of a thermal sensingdevice according to two different embodiments of the present invention.

[0029]FIG. 2 depicts a schematic representation of a thermocouple, saidthermocouple being part of a thermopile.

[0030]FIG. 3 depicts a schematic flow of a method for fabricating athermal sensing device according to an embodiment of the presentinvention.

[0031]FIG. 4 depicts a schematic flow of a method for fabricating arubber membrane, i.e. a membrane composed of . ELASTOSIL LR3003/10A, B(Wacker Chemie), according to an embodiment of the invention.

[0032]FIG. 5 depicts a schematic flow of a method for fabricating athermal sensing device according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] In relation to the appended drawings the present invention isdescribed in detail in the sequel. Several embodiments are disclosed. Itis apparent however that a person skilled in the art can imagine severalother equivalent embodiments or other ways of practicing the presentinvention, the spirit and scope thereof being limited only by the termsof the appended claims.

[0034] A device is disclosed yielding an electrical output signal buthaving an input or intermediate signal of the thermal type. Said devicecan be used e.g. for the analysis of the activity of biological cells,chemical reactions in small volumes and other microanalyticalapplications. Particularly said device can be used to monitor chemicaland physical processes which are accompanied by changes in heat contentor enthalpy. Furthermore said device can be used to thermodynamicallycharacterize a biological interaction as a means to rational drugdesign, to drug stability and drug effect studies on cells and blood.

[0035] In an embodiment of the invention a device, as in FIG. 1, formonitoring chemical and physical processes which are accompanied bychanges in heat content or enthalpy is disclosed, comprising athermopile (2), wherein said thermopile is in contact with as part of asubstrate (1), e.g. a silicon wafer or the remains thereof, and whereinsaid thermopile is at least one thermocouple (FIG. 2) comprising a firstconductive material (14) and a second conductive material (13) with aninsulating layer (15) inbetween. Said first and said second material arechosen such that their thermoelectric voltages are different. The choiceof said conductive materials is further based on several parameters asthere are the magnitude of their Seebeck coefficient, their electricalresistivity, their availability and their compatibility with standardprocessing steps as used in the manufacturing of integrated circuits. Anincrease in Seebeck coefficient corresponds with an increase insensitivity while a lower resistivity corresponds with a lower noiselevel and thus an improved signal to noise ratio. Each thermocouple hasa so-called hot junction (11), where said first material and said secondmaterial are short-circuited, and a so-called cold junction (12), wheresaid first and said second material are separated one from another bymeans of said insulating layer. At said cold junction the electricaloutput signal, representing the temperature difference between said hotjunction and said cold junction of the same thermopile, can be measured.Preferably said thermocouples of said thermopile are connected inparallel thermally and in series electrically. By doing so, the hotjuntions as well as the cold junctions of the different thermocouples ofthe same thermopile are grouped.

[0036] A first substance, i.e. a reference substance, can be thermallycoupled to the cold junctions of said thermopile while a secondsubstance, i.e. a test substance, can be thermally coupled to the hotjunctions of the same thermopile. Alternatively, a first substance, i.e.a test substance, can be thermally coupled to the cold junctions of saidthermopile while a second substance, i.e. a reference substance, can bethermally coupled to the hot junctions of the same thermopile. Saidfirst substance and said second substance can be brought in directcontact with said cold junctions and said hot junctions respectivelyusing supply means, whereby said first substance is isolated from saidsecond substance. Alternatively, said device can further comprise afirst and a second channel on said thermopile, a first channel (3) inclose vicinity of the cold junctions and a second channel (4) in dosevicinity of the hot junctions, said first channel and said secondchannel being separated and isolated one from another. Said channels canbe used to supply the substances, e.g. solutions, to said junctions.Furthermore said channels can also serve as molds for fitting inreplaceable tubes, particularly replaceable glass tubes, wherein saidtubes are used as supply means. Said tubes have a diameter typically inthe micrometer range and can be made by micromachining techniques or canbe readily bought.

[0037] Said device can further comprise a thin insulating layer, e.g. anoxide layer or a nitride layer, on said thermopile in order to prevent adirect contact between the substances and the thermopile to therebyavoid damaging said thermopile.

[0038] Said device further comprises a membrane (5) to isolate andmechanically support said thermopile. Said membrane should thermally andelectrically isolate said thermopile and mechanically support saidthermopile. Silicon oxide and/or silicon nitride can be used as membranematerials. Particularly a liquid rubber, i.e. ELASTOSIL LR3003/10A, Bcan be used as a membrane material. The rubber membrane fulfills thestringent biocompatibility requirements necessary for medicalapplications, allows for relatively large pressures to be build up, e.g.when the substance is a solution which is pumped through the device,renders the sensor excellent thermal isolation properties, enables theactive area to be very large and makes it possible to have opticalaccess thanks to its transparency. Alternatively, instead of a membranean insulating support plate, e.g. a glass plate or a polyvinylchloride(PVC-C) plate can be used.

[0039] To speed up measurement time or to test a number of substances atthe same time, a modular system comprising an array of devices, eachdevice comprising one thermopile, can be configured on the samesubstrate.

[0040] According to the present invention an example of a device (FIG.1, b)) is disclosed based on only one thermopile (2) wherein saidthermopile is in contact with a part of a substrate (1), e.g. a siliconwafer or the remains thereof. The cold junctions of said thermopile arecoupled thermally to a first channel (3) comprising a first substancewhile the hot junctions of said thermopile are coupled thermally to asecond channel (4) comprising a second substance, said first and saidsecond channel are separated and thermally isolated one from another.The device further comprises a polyvinylchloride (PVC-C) support plate(6) at the bottom side and a similar plate (7) is used for ceiling thechannels on the top side. The classical heat sink is omitted therebyfully exploiting the inherent benefits of a differential measurementusing thermocouples as temperature sensing elements and resulting in areduction in overall dimensions. Less area is consumed and the deadvolume of the feeding fluid channels is reduced. Said device is capableof handling a very small amount of a substance, typically in the rangefrom 1 microliter to 30 microliter. When used e.g. for drug screening,reference cells, forming a first substance, are adhered in the firstchannel, while in the second channel genetically engineered cellsexpressing a drug target are cultivated. When the potential drugcandidate is effective, it will activate the genetically engineeredcells, which form said second substance, and these will cause a heatchange at one side of the thermopile, i.e. the hot junctions, therebyproducing a differential voltage (heat conduction type calorimeter). Thereference sample is thus substantially equal to the test sample exceptthat it cannot produce a temperature change due to physiologicalactivation. Consequently heat capacities and surface relationships areequal. The device as such has a very high common mode rejection ratio,offering a signal which is originating only from the potential drugcandidate stimulating or suppressing the metabolism of the cells understudy. To handle living cells and sticky reagents, materials where thedevice is composed of and which are in contact with said cells, need tobe fully biocompatible and sterilisable.

[0041] According to the present invention an example of a method isdisclosed for fabricating a device used to monitor chemical and physicalprocesses which are accompanied by changes in heat content or enthalpy.The device is capable of handling a very small amount of a substance.These requirements can be achieved by micromachining, a techniqueclosely related to integrated circuit fabrication technology. Thestarting material is a substrate like e.g. a semiconductive wafer,particularly a monocrystalline silicon wafer, or a slice of aninsulating material, i.e. a glass slice. On this substrate layers can becoated, patterned by means of a sequence of lithographic steps and wetand/or dry etching steps. Such processed substrates can be bonded toeach other or to other materials in order to make three-dimensionalstructures. A possible implementation of such a method or process isdescribed below as an example without limiting the scope of theinvention. The materials, dimensions and process steps mentioned in thisexample can be easily exchanged with equivalents or equivalent steps.

[0042] The process (FIG. 3) starts with the deposition of an oxide layer(FIG. 3, step a)) with a thickness of 470 nm on a first (22) and asecond side (23) of a substrate (21), i.e. a monocrystalline siliconwafer. On the unpolished side, i.e. the first side, a nitride layer (24)with a thickness of 150 nm is deposited (FIG. 3, step b)) while on theother side, i.e. the second side, a first conductive layer (25), i.e. ap-type doped polysilicon layer with a thickness of about 1 μm isdeposited. Said oxide layer on said second side of the substrate isprovided as an etch stop layer for a back etch, as a layer whichinhibits a direct contact of a substance to a thermopile and tothermally and electrically isolate said first conductive layer, beingpart of a thermopile.

[0043] On said first side of the substrate the nitride/oxide stack ispatterned (FIG. 3, step c)) in order to define the etch windows foretching away the underlying silicon to form twin channels. Said channelsare used to bring substances in close vicinity or in direct contact withthe junctions of a thermopile. On said second side of the substrate thep-type doped polysilicon layer is patterned (FIG. 3, step c)) to therebyform the first material of the thermopile, i.e. a set of thermocoupleswhich are connected in parallel thermally and in series electrically.

[0044] On said second side of the substrate an insulating layer (26) isdeposited (FIG. 3, step d)) which is used as an interconductive layerdielectric and which isolates the different fingers of the polysiliconpattern. Said insulating layer is patterned to thereby form via holesthrough which the underlying polysilicon layer can be contacted in orderto form hot junctions. A second conductive layer (27), having athermo-electric voltage different from the thermo-electric voltage ofsaid first conductive layer, i.e. an aluminum layer with a thickness of200 nm, is deposited by means of evaporation on said second side of thesubstrate. Said second conductive layer is patterned to thereby form thesecond material of the thermopile. Aluminum and p-type polysilicon canbe used to fabricate the thermopile because they are standard materialsand their Seebeck coefficient is large. The dielectric layer between theconductive layers of the thermopile is a photosensitive resin derivedfrom B-staged bisbenzocyclobutene (BCB) monomers.

[0045] The substrate is diced and said second side of the substrate isglued on a polyvinylchloride (PVC-C) support plate (28) before the backetch is done in KOH (FIG. 3, step e)). A similar plate (29) is used forceiling (FIG. 3, step f)) the channels on the top side.

[0046] Instead of said support plate a membrane can be introduced indevices, which are used to monitor chemical and physical processes whichare accompanied by changes in heat content or enthalpy and whichcomprise at least one thermopile, to thermally and electrically isolatesaid thermopile and to mechanically support said thermopile. Whenmembranes larger than a few square centimeters need to be fabricated,conventional micromachining techniques have limitations. The materialswhere conventional membranes are composed of e.g. silicon oxide and/orsilicon nitride. Due to residual stress in these silicon oxide and/orsilicon nitride layer(s) which form the membrane, they easily bend,crack or even break. Therefore several polymers, particularly siliconerubber, have been investigated to make flexible large area membranes.The silicone rubber used is the two-component liquid silicone rubberELASTOSIL LR3003/10A, B (Wacker Chemie). This rubber has a highmechanical strength, i.e. a tensile strength of about 2.5 MPa, asuperior elongation at break of about 620%, a perfect biocompatibility,a low thermal conductivity of about 0.2 W/mK, a high electricalresistivity of about 5.10¹⁵ Ωcm, a low water uptake, a high gaspermeability and a relatively low viscosity. The latter property makes aspincoating technique feasible.

[0047] The biocompatibility, high mechanical strength, high degree oftransparency and low thermal conductivity of this silicone rubber inviteto many application domains where conventional micromachining techniquesfail. This silicone rubber can be introduced as a membrane in sensingdevices comprising a thermopile. Transferring a thermopile to a rubbermembrane renders excellent thermal isolation properties to thethermopile as the thermal conductance of the rubber is very small (0.2W/mK) and the membrane can be made very thin (μm range). Moreover, itoffers the possibility to prepare a large size thermopile which isneeded if good thermal isolation and high sensitivity are desired. Largeareas are also needed if the metabolism of biological cells is beingtested as cells are preferably tested in monolayers and a large numberof them are needed to get a significant signal (power production of asingle cell is in the order of picoWatts). Furthermore the rubbermembrane fulfills the stringent biocompatibility requirements, whichmakes it suited for medical applications, and can sustain relativelylarge pressures. The use of this silicone rubber is not limited to itsfunction as a membrane in sensing devices comprising a thermopile. Duethe its high thermal resistivity this rubber can be used to thermallyisolate at least parts of all kind of sensing devices. Due to itsmechanical strength and elasticity this rubber can be used in all kindof sensing devices which benefit from these properties like e.g. flowsensing devices and actuators. Due to its low viscosity this rubber canbe introduced in sensing devices by means of a spin coating techniquefor protection, sealing and packaging purposes. Furthermore, thetransparency of the rubber opens the field for applications whereoptical access is needed, e.g. microscopic analyzing techniques.

[0048] In an embodiment of the invention a method (FIG. 4) is disclosedfor fabricating rubber membranes. This method comprises the followingsteps:

[0049] On a first side of a substrate (31) a silicon oxide/siliconnitride stack is deposited (FIG. 4, step a)) which will serve as an etchmask to define the membrane pattern. The oxide layer (32) has athickness of 470 nm, while the thickness of the nitride layer (33) is150 nm. Other materials and/or other thickness and or another number oflayers may be used to serve as an etch mask. When using an oxide/nitridestack, preferably the ratio of the thickness of the oxide and thenitride layer is about three to balance out the tensile and compressiveforces. Said substrate can be a semiconductive wafer or slice, like e.g.a silicon wafer, or an insulating slice like e.g. a glass slice.Particularly, a six inch p-type (100) oriented monocrystalline siliconwafer is used.

[0050] On the second side of the substrate a silicon oxide/siliconnitride stack is deposited which will serve as an etch stop to definethe membrane pattern. The oxide layer (34) has a thickness of 470 nmwhile the thickness of the nitride layer (35) is 150 nm. Other,preferably insulating, materials and/or other thickness and or anothernumber of layers may be used to serve as an etch stop. When using anoxide/nitride stack, preferably the ratio of the thickness of the oxideand the nitride layer is about three to balance out the tensile andcompressive forces. One can also choose to omit this etch stop dependenton the etch procedure.

[0051] The oxide/nitride stack on said first side of the substrate ispatterned (FIG. 4, step b)) by means of a sequence of photolithographicsteps and wet and/or dry etching steps.

[0052] The second side of the substrate is coated with liquid rubber,i.e. ELASTOSIL LR3003/10A, B (Wacker Chemie) (36). The relatively lowviscosity of said rubber allows for a spin-coating technique. By varyingthe speed and the time of spinning, the thickness of the layer can beadjusted ranging from 5 to 50 μm. For larger thickness, a multilayerstructure can be fabricated by spinning different layers on top of eachother. A spinrate of 3000 rpm and a spintime of 60 seconds renders alayer thickness of about 70 μm. The surface of the substrate ischemically modified to make it water repellent by treating said surfacewith hexamethyldisiloxane (HMDS). The viscosity of the rubber, and thusthe layer thickness, can be reduced by adding small amounts of siliconeoil.

[0053] A second substrate (37), particularly a second 6 inch wafer isbonded (FIG. 4, step c)) onto the first by means of the unvulcanisedrubber. The bonding is performed in low vacuum to avoid air bubbleformation at the substrate-rubber interface. To cure the rubber, thestructure is baked for 3 minutes at 170° C. on a hot plate.Alternatively, in stead of a second wafer a glass plate is used. A firstside of this glass plate comprises a wax layer to protect the rubberlayer of the first substrate because said first side is exposed to therubber during the bonding process.

[0054] To form the membrane a chemical back etch (FIG. 4, step d)) isperformed in 35 w % KOH at 60° C. The last 10 μm of silicon is etched atroom temperature to minimize the risk of breaking the oxide/nitridelayer underneath. The rubber type used is not attacked by KOH at roomtemperature so the second sacrificial substrate needs no etch stop. Ifthe bottom oxide/nitride layer of the first substrate is not astructural element of the design, it be omitted from the beginning,making the rubber layer an etch stop for both etching sides. Theoxide/nitride stack is necessary in cases where e.g. gas impermeabilityis needed (the rubber is permeable for gasses) or chemicals need to betransported which attack the rubber. In case a waxed glass plate is usedinstead of a silicon wafer, the second side of said plate is etched inKOH at 40° C. The wax is removed by 1,1,1-trichloroethane.

[0055] In an embodiment of the invention a method is disclosed forfabricating a device used to monitor chemical and physical processeswhich are accompanied by changes in heat content or enthalpy. The devicecan be capable of handling a very small amount of a substance. Theserequirements can be achieved by micromachining, a technique closelyrelated to integrated circuit fabrication technology. The startingmaterial is a substrate like e.g. a semiconductive wafer or slice,particularly a monocrystalline silicon wafer, or a slice or plate of aninsulating material, i.e. a glass slice. Particularly a 150 mm siliconwafer is chosen. This method or process comprises the following steps(FIG. 5):

[0056] On a first side of a substrate (51) at least one hard mask layer(52) is deposited (FIG. 5, step a)) which will serve as an etch mask forremoving at least parts of said substrate. An example of said hard masklayer is a multi-layer structure comprising a first layer e.g. siliconoxide and a second layer e.g. a silicon nitride. Other materials and/oranother number of layers may be chosen dependent on their suitability asan etch mask. When choosing for an oxide/nitride stack, preferably theratio of the thickness of the oxide and the nitride layer is about threeto balance out the tensile and compressive forces. This results in anoxide layer with a thickness which is typically about 450 nm while thethickness of the nitride layer is typically about 150 nm.

[0057] On the second side of the substrate at least one insulating layer(53) can be deposited which can serve as an etch stop layer dependent onthe etch procedure used and/or as an insulating layer to thermally andelectrically isolate a thermopile and/or to inhibit a direct contactbetween a substance and said thermopile. An example of said hard masklayer is a multi-layer structure comprising a first layer e.g. a siliconoxide layer and a second layer e.g. a silicon nitride layer. Otherinsulating materials and/or another number of layers may be chosendependent on their suitability as an etch stop. When choosing for anoxide/nitride stack, preferably the ratio of the thickness of the oxideand the nitride layer is about three to balance out the tensile andcompressive forces. This results in an oxide layer with a thicknesswhich is typically about 450 nm while the thickness of the nitride layeris typically about 150 nm. One can also choose to omit at least a partof this multi-layer structure when its only function is to provide anetch stop, dependent on the etch procedure used.

[0058] On said second side of the substrate a first conductive layer(54) with a thickness typically in the range from 0.3μ/m to 1 μm isdeposited. The choice of said conductive layer is based on severalparameters as there are the magnitude of its Seebeck coefficient, theelectrical resistivity, its availability and the compatibility withstandard processing steps as used in the manufacturing of integratedcircuits. An increase in Seebeck coefficient corresponds with anincrease in sensitivity while a lower resistivity corresponds with alower noise level and thus an improved signal to noise ratio. Theresistance of said conductive layer is of course not only determined byits resistivity but also its dimensions which is at least partly adesign issue. An example of such a conductive layer is a dopedpolysilicon layer. In case a polysilicon layer is chosen, said layer isdoped after deposition.

[0059] On said first side of the substrate the nitride/oxide stack ispatterned (FIG. 5, step b)) in order to define the etch windows foretching away the underlying silicon in order to expose at least parts ofthe underlying thermopile or the etch stop multilayer structure on saidthermopile. This can be done to form channel regions in the vicinity ofthe thermopile junctions. These channel regions can be used to bring atest or reference substance in contact or in the vicinity of thejunctions of said thermopile. These channel regions can also be used asmolds for fitting in e.g. glass tubes, where said glass tubes are usedto bring a test or a reference substance in the vicinity of saidthermopile thereby avoiding a direct contact between said substance andsaid thermopile. Alternatively, in stead of etching channel regions, onecan also choose to expose the underlying thermopile or the etch stopmulti-layer structure on said thermopile as a whole. In the latter casethe reference substance or the test substance is applied directly onsaid thermopile.

[0060] On said second side of the substrate said first conductive layer(54), e.g. a doped polysilicon layer is patterned to thereby form thefirst material of the thermopile, i.e. a set of thermocouples which arepreferably connected in parallel thermally and in series electrically.

[0061] On said second side of the substrate an insulating layer (55) isdeposited (FIG. 5, step c)) with a thickness in the range typically from0.2 μm to 1 μm or from 0.5 μm to 5 μm. Said insulating layer is used asan interconductive layer dielectric and isolates the different fingersof the polysilicon pattern from each other. Said insulating layer ispatterned to thereby form via holes through which the underlying firstconductive layer can be contacted in order to form hot junctions.Examples of such insulating layers are a silicon oxide layer and abenzocyclobutene (BCB) layer.

[0062] A second conductive layer (56), having a thermo-electric voltagedifferent from the thermoelectric voltage of said first conductivelayer, i.e. an aluminum layer with a thickness of 200 nm, is depositede.g. by means of evaporation on said second side of the substrate. Saidsecond conductive layer is patterned to thereby form the second materialof the thermopile.

[0063] On said second side of the substrate an insulating layer (57) isdeposited (FIG. 5, step d)) to serve as a membrane. Said membrane shouldthermally and electrically isolate said thermopile and mechanicallysupport said thermopile. Silicon oxide and/or silicon nitride can beused as membrane materials, but preferably a liquid rubber, i.e.ELASTOSIL LR3003/10A, B (Wacker Chemie) is used. The rubber membranefulfills the stringent biocompatibility requirements necessary formedical applications, allows for relatively large pressures to be buildup, e.g. when the test substance is a solution which is pumped throughthe device, renders the sensor excellent thermal isolation properties,enables the active area to be very large and makes it possible to haveoptical access thanks to its transparency. The relatively low viscosityof said rubber allows for a spin-coating technique. By varying the speedand the time of spinning, the thickness of the layer can be adjustedranging from 5 to 50 μm. For larger thickness, a multilayer structurecan be fabricated by spinning different layers on top of each other. Aspinrate of 3000 rpm and a spintime of 60 seconds renders a layerthickness of about 70 μm. The surface of the substrate is chemicallymodified to make it water repellent by treating said surface withhexamethyldisiloxane (HMDS). The viscosity of the rubber, and thus thelayer thickness, can be further reduced by adding small amounts ofsilicone oil resulting in layer thickness down to 1 μm.

[0064] A second substrate (58), particularly a second 150 mm siliconwafer, is bonded (FIG. 5, step e)) onto said first substrate by means ofthe unvulcanised rubber. The bonding is performed in low vacuum to avoidair bubble formation at the substrate-rubber interface. To cure therubber, the structure is baked for 3 minutes at 170° C. on a hot plate.Alternatively, in stead of a second wafer a glass plate is used. A firstside of this glass plate comprises a wax layer to protect the rubberlayer of the first substrate because said first side is exposed to therubber during the bonding process.

[0065] To expose the thermopile and the membrane, a chemical back etchis performed (FIG. 5, step f)) in 35 w % KOH at 60° C. The last 10 μm ofsilicon is etched at room temperature to minimize the risk of breakingthe oxide/nitride layer on said thermopile. The rubber type used is notattacked by KOH at room temperature so the second sacrificial substrateneeds no etch stop. In case a waxed glass plate is used in stead of asilicon wafer, the second side of said plate is etched in KOH at 40° C.The wax is removed by 1,1,1-trichloroethane. Alternatively an etch mask(59), e.g. a silicon oxide/silicon nitride stack, can be deposited andpatterned on the free surface of said second substrate. By doing so,during the back etch e.g. only the glass or silicon underneath thethermopile is removed to thereby free the membrane.

[0066] Alternatively, instead of forming a membrane on said thermopileone can also choose to fix a substrate, like e.g. a glass plate or apolyvinylchloride plate, directly on said thermopile. Said substratewill isolate and mechanically support said thermopile.

What is claimed is:
 1. A method for detecting an effect of a cellaffecting agent on a cell comprising: providing a device, said devicecomprising a support; a membrane having a first surface and a secondsurface, said membrane mechanically supported by said support on atleast one point on one of said first surface or said second surface, atleast one pair of devices, wherein one device of said pair of devices islocated in a first region of said first surface of said membrane and theother device of said pair of devices is located in a second region ofsaid first surface of said membrane, wherein said membrane thermally andelectrically isolates said first region and said second region, andwherein each device of said pair of devices yields an electrical outputsignal in response to having an input signal of the thermal type, afirst supply for bringing a first substance in thermal contact with saidfirst region, and a second supply for bringing a second substance inthermal contact with said second region, wherein said first substance isseparated and thermally isolated from said second substance; providingreference cells in said first region; providing genetically modifiedcells in said second region, said genetically modified cells expressinga cell affecting agent target; contacting said cells in the first regionand the second region with said cell affecting agent; and determiningwhether there is a relative heat change in said second region.